1. Field of the Invention
This invention relates to a device for driving an ink jet head for applying an electric field in the vertical direction to the polarizing direction to a piezoelectric element to cause displacement of a shear mode, and pressurizing the ink by this displacement to discharge ink selectively from plural nozzles and form an image on a paper sheet and particularly to the ink jet head driver, which may effectively perform precursor microvibration (the operation of previously vibrating meniscus of ink to such an extent not to discharge ink from the nozzles) to improve intermittent discharge performance in the ink jet head.
2. Description of Related Art
One structural example of a shear-mode ink jet head will be described. As disclosed in the patent document 1 mentioned later, in a two-layer planar structure where two piezoelectric elements polarized in the opposite directions to each other with respect to the direction of board thickness are bonded by an adhesive, the adhesive surfaces of the piezoelectric elements are cut from one surface of the piezoelectric element so that a number of grooves at fixed intervals pass, the tip parts of the respective grooves are opened at the plate to form a comb-like structure, the top sides of these grooves being closed by another plate. The rear ends of the grooves are communicated with a common ink chamber, the openings at the tip parts are provided with an orifice plate having a nozzle in a position of each groove, and an electrode is provided on the inner surface of the groove.
In the above construction, when an electric field in the vertical direction to the polarizing direction of the electrode is applied to the piezoelectric elements, displacement of the shear-mode is cased in the piezoelectric member, and the ink in the grooves is pressurized by this displacement so that ink drops are discharged from the selected nozzles.
In an image forming apparatus using the thus constructed ink jet head, generally the ink jet head is fixedly provided in a predetermined position so that a plurality of nozzles are arranged in the direction orthogonal to the transport direction of a paper sheet, the paper sheet is transported thereto by a transport mechanism, and in synchronization with the transport of the paper sheet, the ink jet head is driven in a suitable timing, thereby making a discharged ink drop to the paper sheet to form a desired image. Concerning the respective pixels constituting an image to be formed, the maximum of drops to be discharged is predetermined, and the ink drops of a number required by each pixel corresponding to the size of the concerned pixel within the range are discharged.
In the above shear-mode ink jet head, as shown in the following patent document 2, in some case, precursor microvibration is performed in order to improve the intermittent discharge performance of ink drops, and in that case, it is effective to perform the precursor microvibration immediately before image formation.
In the image formation using the shear-mode ink jet head, as to what is the timing of performing precursor microvibration, according to the prior art, mostly in the image formation process conducted by driving the ink jet head while the paper sheet is transported to the ink jet head, the precursor microvibration is performed for all of the nozzles outside of the image formation range of the paper sheet, and even for the nozzles within the image formation range of the paper sheet, that actually do not discharge ink drops for image formation, the precursor microvibration is always performed.
[Patent Document 1] Japanese Publication Number of Unexamined Application: 2000-135787
[Patent Document 2] Japanese Publication Number of Unexamined Application: 2004-230775
In the image forming apparatus using the above shear-mode ink jet head, however, since the head is fixed and the paper sheet is transported as described above, even if the precursor microvibration is performed for all of nozzles immediately before image formation, with the nozzle adapted to discharge a small dot only to the tailing end of the paper sheet, when the small dot is formed into an image after the precursor vibration is performed, several seconds have already elapsed after the latest precursor microvibration in some case depending on the paper size and the paper transport speed. In that case, discharge failure is caused.
In the nozzle which discharges ink drops of a number under the maximum of discharged drops, when the precursor microvibration is always performed in the timing of non-discharge, there is a fear of a temperature rise of the head.